Functional Redundancy (Part 1): What Ecologists Mean By “Diversity"

This is part of our sequence of blogs about “functional redundancy”, a concept from ecology that explains why diverse ecosystems are often more productive and resilient. The following series explores this history of the concept of functional redundancy, why it matters to agriculture, and how we can use it to build better biological products that function consistently across environments.

First, what is ecology?

"Ecology" gets used loosely, often as shorthand for "environmental" or "natural" which doesn’t truly explain it. Really, ecology is a more specific concept and could be summarized as: how living things interact with each other and their surroundings. Another way to say it is that ecology is about relationships: who eats whom, who competes with whom, who depends on whom, and how those interactions add up to the things like productive grasslands, clean water, and fertile soil.

Each of these examples can be considered an ecosystem, and a useful way to think about an ecosystem is as a workplace full of different "jobs". Some organisms capture sunlight. Some break down dead material. Some cycle nitrogen, some move nutrients, some keep other populations in check. When ecologists talk about function, they mean these ecosystem "jobs" and how those jobs are getting done.

The diversity–stability question

For much of the 20th century, ecologists have argued over a simple-sounding question: do ecosystems with more species hold up better than those with fewer? The diversity–stability hypothesis traces back to proposals by Odum (1953), MacArthur (1955), and Elton (1958), and was followed on by evidence that diversity does tend to produce stability.

Some of the most influential evidence came from Minnesota. David Tilman's widely cited 1994 paper drew on an experiment begun in 1982 with more than 200 grassland plots at the Cedar Creek Ecosystem Science Reserve. When the 1988 drought hit, it revealed a strong positive link between plant diversity and the stability of ecosystem productivity. Tilman and Downing reported that productivity in more diverse plant communities resisted the drought better and recovered more completely afterward. The drought functioned as an accidental experiment, and diversity was insurance.

Enter functional redundancy

If diversity helps with productivity, the next question is why. Two key models arose to try to answer this question.

In a 1981 model (Erlich), you can picture species in an ecosystem as rivets holding an airplane wing together. You could pop a few and the wing would hold. Keep popping, though, and at some point you lose the wing, and can't necessarily predict which rivet will be the last one. Then, in 1992 (Walker), in what became known as the "redundancy hypothesis", it was argued most species behave more like passengers than rivets, and only a few key "drivers" are needed to keep the plane flying. He proposed organizing the problem around functional groups of organisms, defined by the ecosystem processes they carry out.

That's the core definition of functional redundancy: more than one species capable of doing the same job. If one nitrogen fixer disappears and another can take over, the function survives.

Where "diversity" and "redundancy" overlap, and where they don't

These two ideas are related but not identical, and the difference is worth spelling out.

Diversity counts who is there: how many species, how evenly distributed, how different from one another.

Functional redundancy asks how many of them can do each job.

You can have a very diverse ecosystem that is thin on redundancy for one critical function. That's lots of species overall, but only one or two that do a particular job. You can also have modest diversity but plenty of backup for a single function. A high species count doesn't by itself tell you whether a system is protected, but, the two are often related.

However, Tilman and Downing's drought results found that each species lost from the grassland had a progressively larger effect on resistance, and so concluded their data supported the diversity–stability hypothesis but not the idea that most species are exactly functionally redundant. This is an important nuance: redundancy isn't necessarily a pile of spare parts you can afford to lose. Species that look interchangeable under normal conditions may turn out to matter a great deal when conditions change. Why?

What matters: not only diversity of organisms, but also diversity of Mechanisms of Action (MoAs)

Two organisms might do the same job, but they rarely do it under exactly the same conditions or the same way. One nitrogen-cycling microbe may thrive in warm, wet soil; another may carry the load when it turns cool and dry. In terms of functional groups they look the same, both N-fixers, both depositing atmospheric nitrogen to the rhizosphere. But in practice they are complementary: each covers for the other when conditions shift.

Ecologists formalized these ideas in two steps. First came the “insurance” hypothesis. As Yachi and Loreau described it in 1999, biodiversity insures ecosystems against declining function because having many species raises the odds that some will keep functioning even if others fail. They observed the benefit of biodiversity to occur in two main ways: 1) an increase in average productivity and 2) a “buffering” effect that reduces how much productivity fluctuates over time. Elmqvist et al. coined the term "response diversity" in 2003 for the range of ways species contribute the same function and argued it is critical to ecosystem resilience, renewal and reorganization after disturbance.

Put simply: redundancy means several species can do a job. Response diversity means they aren’t all vulnerable to fail at once. This is where the real functionality lives.

References

• Fiegna, F. et al. (2015). Evolution of species interactions determines microbial community productivity in new environments. The ISME Journal 9: 1235–1245.

• Tsiafouli, M. A. et al. (2015). Intensive agriculture reduces soil biodiversity across Europe. Global Change Biology 21: 973–985.

• Wei, Z. et al. (2015). Trophic network architecture of root-associated bacterial communities determines pathogen invasion and plant health. Nature Communications 6: 8413.

• (2022). Species interactions constrain adaptation and preserve ecological stability in an experimental microbial community. The ISME Journal.

• Ehrlich, P. & Ehrlich, A. (1981). Extinction: The Causes and Consequences of the Disappearance of Species. Random House.

• Elmqvist, T. et al. (2003). Response diversity, ecosystem change, and resilience. Frontiers in Ecology and the Environment 1: 488–494.

• May, R. M. (1973). Stability and Complexity in Model Ecosystems. Princeton University Press. Tilman, D. & Downing, J. A. (1994). Biodiversity and stability in grasslands. Nature 367: 363–365.

• Walker, B. H. (1992). Biodiversity and ecological redundancy. Conservation Biology 6: 18–23.

• Yachi, S. & Loreau, M. (1999). Biodiversity and ecosystem productivity in a fluctuating environment: the insurance hypothesis. PNAS 96: 1463–1468.

• Allison, S. D. & Martiny, J. B. H. (2008). Resistance, resilience, and redundancy in microbial communities. PNAS 105 (Suppl. 1): 11512–11519.

• Louca, S. et al. (2018). Function and functional redundancy in microbial systems. Nature Ecology & Evolution 2: 936–943.

• Mendes, R. et al. (2011). Deciphering the rhizosphere microbiome for disease-suppressive bacteria. Science 332: 1097–1100.

• Wei, Z. et al. (2015). Trophic network architecture of root-associated bacterial communities determines pathogen invasion and plant health. Nature Communications 6: 8413.